my fault guys, its a 2000 klr 650. the stock air filter was pretty dirty, i dont think i should put that thing back in it. im wondering if i should downsize the jet size to the 140 instead of the 150. and i was thinking maybe doing the "L" mod to the air filter would help it out some. but what the hell do i know?!
Well, snuffy, popping on decel is an indication a lean condition, as you know.
Doing the L-Mod will (theoretically) allow more air into the engine, leaning the mixture, so that's probably not what you want to do at this point.
As to the 150 jet, I just don't know what the Studebaker stuff is. See, a 150 jet ain't a 150 jet all the time. The thing about jets is that everybody has their own standard. Kinda defeats the purpose of standards....
A 150 Keihin jet would be awfully rich - there's no configuration on the KLR that would warrant it. The head just won't flow enough, no matter what you do to the exhaust and intake, to use that much jet. Well, maybe a 685 kit, ported head, big valves and a really free flowing pipe...nah, doubt even then.
You've got several things going that are somewhat contradictory. You put on an exhaust, which ought to flow better, leaning the mixture. But you've installed some sort of 150 jet, which may be bigger than stock (probably is) but you've also installed a needle that we don't know the profile of. It's hard to judge what the net effect is.
The K&N is reputed to be more restrictive than the stock, so that would richen things up a bit (I bet you could clean that old stock filter!).
You've also turned the mixture screw out, which should richen the idle circuit, but you've still got popping.
My advice would be to put the stock jet and needle back in, but with two stainless #4 washers under the head.
Turn the mixture screw in until it is gently seated, then back it out 2 turns.
See how things are working then, and we can go from there.
Now, on that jet and needle - do you have anyway to measure (pin gauges) the size of the 150 jet? Or did the jet kit come with a table that tells you what size it is? Do you have a way to chart out the taper of the needle that came with the kit? I can provide you with a table of Keihin jet sizes and the taper of the stock needle and of a KLX needle for reference. That may come in handy for later tuning once you get that popping under control.
A word on the idle circuit. I have made what I consider to be pretty extreme mods to my intake and exhaust. I found that the stock idle jet to be too small for my mods and had to go up a size. Resist the urge to solve your problem by going to a larger idle jet. I can tell you that your mods have to be pretty extreme to need that. If you do such a thing (not saying you're contemplating it!) you might solve the problem, but you've really just masked what the real problem is.
Do you have a way to check the air/fuel ratio? If you have good records of gas mileage, you can make changes and check the gas mileage to see if it's gotten better or worse - slow, but effective. Are you familiar with reading plugs? Maybe have a wideband O2 meter? To make this all work right you're going to need to be fairly certain that your mixture is correct.